1. Field of the Invention
The oiling of vegetable- and/or mineral-tanned leathers and skins is an essential step in the finishing process leading to the ready-to-use material. The form in which the oil is distributed in the skin and the extent to which the oil components are fixed in the skin critically influence the properties and performance of the end products. Extensive expert knowledge exists on possible interactions between the oil components on the one hand and the tanned skin containing residual tanning agents on the other hand. The particular composition of the oiling preparations--for example the number of lipophilic groups and reactive groups present, if any, for reaction with suitable reactive constituents in the tanned leather--determine inter alia the permanence and effectiveness of the oil finish in the practical use of the leathers and skins.
An important practical aspect lies in the provision of oil finishes which can be so reliably fixed in the tanned skin that the leathers and skins are sufficiently resistant to washing and cleaning for practical requirements. High-quality leather goods, for example from the clothing industry, are expected to lend themselves both to washing with water/detergents and also to dry cleaning without any significant loss of quality. There are also special cases where the finished leather is required to show sufficient impermeability to water.
2. Discussion of Related Art
In principle, there are in particular three known processes for the waterproof finishing of leather and skins:
1. impregnation by incorporation of water-insoluble compounds, for example solid fats, waxes or special polymers, PA1 2. impregnation by incorporation of water-swelling compounds which form highly viscous emulsions on taking up water and which block the fiber interstices of the leather, for example special emulsifiers of the w/o type, PA1 3. treatment with hydrophobicizing compounds, for example aluminium, chromium and/or zirconium complexes, silicones or organic fluorine compounds. PA1 (a) semiesters of maleic acid with oleophilic alcohols and/or lower alkylene oxide adducts thereof and PA1 (b) acrylic acid and/or methacrylic acid PA1 (c) small quantities of other hydrophilic and/or oleophilic comonomers in the oligomer molecule,
DE 1 669 347 describes the use of water-emulsifiable sulfosuccinic acid semiesters for oiling leather, although no waterproof effects are obtained. EP 193 832 relates to a process for the production of waterproof leathers and skins using sulfosuccinic acid monoesters in combination with impregnating and/or hydrophobicizing oiling preparations, the process being characterized in that, after retanning, the leathers and skins are treated in an aqueous liquor with impregnating and/or hydrophobicizing oiling preparations containing sulfosuccinic acid monoester salts with C.sub.12-24 fatty groups and, after acidification, are fixed by addition of a chromium, zirconium and/or aluminium salt. The sulfosuccinic acid monoester salts are preferably used with impregnating oiling preparations from the group of oxidized or oxidized and partly sulfonated C.sub.18-26 hydrocarbons and C.sub.32-40 waxes, phosphoric acid mono-C.sub.12-24 -alkyl esters, citric acid mono-C.sub.16-24 -alkyl esters, sorbitan, glycerol and/or pentaerythritol C.sub.16-24 fatty acid esters.
Amphiphilic preparations in the form of selected co-oligomers of, on the one hand, hydrophobic or oleophilic monomers and, on the other hand, hydrophilic monomers have recently been described for the oiling of, in particular, mineral-tanned leathers and skins. Amphiphilic preparations of this type may be incorporated, for example by milling, in the leathers or skins to be finished in the form of aqueous dispersions, emulsions and/or solutions on completion of the main tanning process. In the case of mineral-tanned leathers or skins in particular, the amphiphilic preparations may also perform the retanning function. Finally, the amphiphilic preparations may be fixed, more particularly with mineral tanning agents. The more recent patent literature describes auxiliaries of the type in question. For example, EP 372 746 describes corresponding preparations and their use, the amphiphilic copolymers consisting predominantly of at least one hydrophobic monomer and, to a lesser extent, of at least one copolymerizable hydrophilic monomer. The hydrophobic monomers listed include long-chain alkyl (meth)acrylates, long-chain alkoxy or alkylphenoxy (polyethylene oxide) (meth)acrylates, primary alkenes, vinyl esters of long-chain alkyl carboxylic acids and mixtures thereof. The hydrophilic comonomers forming the minor component of the copolymers are ethylenically unsaturated water-soluble acids or hydrophilic basic comonomers. The molecular weight (weight average) of the copolymers is in the range from 2,000 to 100,000.
EP 412 389 describes the use of copolymers which have been prepared by radical bulk copolymerization of (a) C.sub.8-40 monoolefins with (b) ethylenically unsaturated C.sub.4-8 dicarboxylic anhydrides at temperatures of 80.degree. to 300.degree. C. to form copolymers with molecular weights of 500 to 20,000 g/mole, subsequent solvolysis of the anhydride groups of the copolymers and at least partial neutralization of the carboxyl groups formed during the solvolysis with bases in aqueous medium and which are present in the form of aqueous dispersions or solutions as preparations for hydrophobicizing leather and skins. EP 418 661 describes the use for the same purpose of copolymers which contain (a) 50 to 90% by weight of C.sub.8-40 alkyl (meth)acrylates, vinyl esters of C.sub.8-40 carboxylic acids or mixtures thereof and (b) 10 to 50% by weight of monoethylenically unsaturated C.sub.3-12 carboxylic acids, monoethylenically unsaturated dicarboxylic anhydrides, semiesters or semiamides of monoethylenically unsaturated C.sub.4-12 dicarboxylic acids, amides of C.sub.3-12 monocarboxylic acids or mixtures thereof in copolymerized form and which have molecular weights of 500 to 30,000 g/mole. For the stated application, the copolymers are at least partly neutralized and are present in the form of aqueous solutions or aqueous dispersions.
Studies conducted by applicants have shown that comparatively low molecular weights can be crucially important to the rapid and uniform penetration of oiling compositions of this type. So far as the polymer chemist is concerned, indirect references to this effect can also be found in the last three of the above-cited documents in the description of the production of the copolymers disclosed therein. The radical copolymerization of the claimed components for (a) and (b) is best carried out in the presence of chain transfer agents, such as mercaptans (EP 372 746 A2, page 6, 20/21) and in the presence of regulators, such as C.sub.1-4 aldehydes, allyl alcohol, but-1-en-3-ol, formic acid or organic compounds containing SH groups (EP 418 661 A1, column 6, paragraph 2). The use of corresponding regulators for obtaining polymers of sufficiently low molecular weight is also recommended in EP 412 389, see A1, column 4, paragraph 2.
Applicants' earlier patent application DE-A-P 41 29 244.8 relates to the use of aqueous dispersions of co-oligomers from the radical-initiated aqueous emulsion copolymerization at mildly acidic to neutral pH values of
as principal components which may additionally contain
as amphiphilic preparations for the oiling and water-resistant finishing of leathers and skins.